Estimating sludge application rates to land based on heavy metal and phosphorus sorption characteristics of soil

Abstract

This paper details the results of research which examined the sorption characteristics of a forest soil profile for phosphorus (P) and the heavy metals arsenic (As), cadmium (Cd), copper (Cu), nickel (Ni) and zinc (Zn). It is suggested that these sorption measurements may allow an estimate of metal and P loadings and subsequently sludge application rates which are environmentally acceptable, particularly in terms of minimal off-site movement. The extent of sorption of an element in a particular soil horizon was characterised by sorption curves which showed the relationship between the amount of element sorbed by the soil and the concentration of that element remaining in the supernatant solution after an equilibrating period. Buffer capacity, defined by the slope of the sorption curve, was calculated for each element in each soil horizon. Sorption behaviour of heavy metals and phosphorus varied greatly within the soil profile. Cadmium (Cd), nickel (Ni) and copper (Cu) were retained mainly in the surface All horizon. This was indicated by their relatively higher buffer capacities in this horizon and may be due to their affinity for organic matter which was present in greatest concentrations in this part of the soil profile. On the other hand, the greatest sorption capacity for arsenic (As) and phosphorus (P) occurred deep in the profile in the B2 horizon where there was a predominance of clay and iron (Fe) and aluminium (Al) hydrous oxides. Metal and phosphorus sorption equations were used to estimate sludge-borne elemental loadings to soil at an “allowable” output elemental concentration of soil leachate to the environment. These loadings, together with soil bulk density and extractabilities of elements in the sludge, were incorporated in a “first approximation” model to estimate sludge application rates which were environmentally acceptable in terms of minimal off-site movement. These rates, along with other factors such as health risk and contamination aspects, should be taken into account when calculating ecologically sustainable sludge application rates at a particular disposal site.